The purpose of this study is to identify predictors of mental retardation in a population of 37,000 children followed from the prenatal period to age 7. Risk factors were examined separately for severely and mildly retarded children, and for subgroups without an identified cause or major neurological abnormality. The incidence of mild retardation (1% in whites and 5% in blacks) and to a lesser degree, of severe retardation (0.5%) was inversely related to socio-economic status. Among the severely retarded, 25% of whites and 50% of blacks had no major genetic or neurological abnormality. Perinatal risk factors for the severely retarded groups as a whole include Down's syndrome, major CNS malformations, neonatal seizures and clinical signs of perinatal hypoxia. For the subgroup of severely retarded with unknown etiology, perinatal risk factors include non-CNS malformations, peripheral nerve abnormalities, signs of hypoxia, and maternal urinary tract infection during pregnancy. Special studies of drugs taken during pregnancy and of the incidence of mental retardation in relatives were conducted. This investigation is completed and a monograph is in preparation.